1. Field of the Invention
The present invention relates to a liquid crystal display device including a plurality of sub picture element electrodes in one picture element region, and to a method of preventing image sticking thereon. More specifically, the present invention relates to such a liquid crystal display device in which at least one of the sub picture element electrodes is capacitively coupled with a control electrode to which a display voltage is applied, and to a method of preventing image sticking thereon.
2. Description of the Prior Art
As compared to a cathode ray tube (CRT), liquid crystal display devices have advantages that they are thin, lightweight, as well as have low voltage drive capability and low power consumption. For this reason, liquid crystal display devices are applied to various electronic devices including televisions, notebook personal computers (PCs), desktop PCs, personal digital assistants (PDAs), cellular telephones, and the like. In particular, an active matrix liquid crystal display device provided with thin film transistors (TFTs) as switching elements for respective picture elements (sub pixels) exerts excellent display characteristics almost equal to a CRT owing to high driving performances. Accordingly, active matrix liquid crystal display devices are now used in various fields, such as desktop PCs and televisions, where CRTs have been conventionally applied.
In general, a liquid crystal display device includes two substrates and liquid crystal which is sealed between these substrates. Picture element electrodes, TFTs, and the like are formed for respective picture elements on one of the substrates. Meanwhile, color filters opposed to the picture element electrodes, and a common electrode common to the respective picture elements are formed on the other substrate. The color filters include three types of red (R), green (G), and blue (B), and a color filter in one of these colors is disposed at each of the picture elements. The three picture elements in red (R), green (G), and blue (B), which are disposed adjacently to one another, collectively constitute one pixel. In this specification, the substrate including formation of the picture element electrodes and the TFTs will be hereinafter referred to as a TFT substrate, and the substrate to be disposed opposite to the TFT substrate will be hereinafter referred to as a counter substrate. Moreover, a structure formed by sealing the liquid crystal between the TFT substrate and the counter substrate will be hereinafter referred to as a liquid crystal panel.
Conventionally, a twisted nematic (TN) liquid crystal display device, which is configured to seal horizontal alignment type liquid crystal (liquid crystal having positive dielectric constant anisotropy) between two substrates and to subject liquid crystal molecules to twisted alignment, has been widely used. However, the twisted nematic liquid crystal display device has a disadvantage of a poor view angle characteristic where contrast and color tone vary largely when a screen is viewed from an inclined angle. For this reason, a multi-domain vertical alignment (MVA) liquid crystal display device having a fine view angle characteristic has been developed and put into practical use.
FIG. 1A and FIG. 1B are schematic cross-sectional views showing an example of a MVA liquid crystal display device. A TFT substrate 10 and a counter substrate 20 are disposed opposite to each other while sandwiching spacers (not shown), and vertical alignment type liquid crystal (liquid crystal having negative dielectric constant anisotropy) 30 is sealed between these substrates 10 and 20. Picture element electrodes 12 on the TFT substrate 10 are provided with slits 12a serving as domain regulating structures to determine directions of inclinations of liquid crystal molecules upon application of a voltage. Surfaces of the picture element electrodes 12 are covered with a vertical alignment film 14 made of polyimide, for example.
A plurality of protrusions 23 in the shape of mounds are formed below a common electrode 22 of the counter substrate 20 as domain regulating structures. As shown in FIG. 1A, these protrusions 23 are disposed in positions which are shifted in oblique directions relative to the slits 12a on the substrate 10. Surfaces of the common electrode 22 and the protrusions 23 are also covered with a vertical alignment film 24 made of polyimide, for example.
Polarizing plates (not shown) are disposed below the TFT substrate 10 and above the counter substrate 20, respectively. These polarizing plates are disposed so as to set absorptions axes orthogonal to each other.
In the MVA liquid crystal display device having the above-described configuration, when a voltage is not applied between the picture element electrodes 12 and the common electrode 22, most of liquid crystal molecules 30a are aligned perpendicularly to the surfaces of the substrates. However, the liquid crystal molecules 30a in the vicinity of the protrusions 23 are aligned perpendicularly to inclined surfaces of the protrusions 23. In this case, light entering a liquid crystal layer from a bottom of the TFT substrate 10 through the polarizing plate is transmitted through the liquid crystal layer while not changing the direction of polarization, and is then shielded by the other polarizing plate above the counter substrate 20. In short, a black display is achieved in this case.
When a given voltage is applied between the picture element electrodes 12 and the common electrode 22, the liquid crystal molecules 30a are aligned obliquely to the surfaces of the substrates due to an influence of an electric field. In this case, as shown in FIG. 1B, the directions of inclination of the liquid crystal molecules 30a are different on two sides of each slit 12a or each protrusion 23. In this way, so-called alignment division (or multi-domains) is achieved. When the liquid crystal molecules 30a are aligned obliquely to the surfaces of the substrates as shown in FIG. 1B, the light which enters the liquid crystal layer from the bottom of the TFT substrate 10 through the polarizing plate changes the direction of polarization and is transmitted through the polarizing plate above the counter substrate 20. The amount of the light transmitted through the polarizing plates depends on the voltage applied between the picture element electrodes 12 and the common electrode 22.
In the MVA liquid crystal display device, since the directions of inclination of the liquid crystal molecules 30a are different on the two sides of each slit 12a or each protrusion 23 upon application of the voltage as shown in FIG. 1B. Accordingly, leakage of the light in oblique directions is suppressed and an excellent view angle characteristic is obtained.
Although the above example explains the case where the protrusions and the slits constitute the domain regulating structures, there is also a case where recesses (grooves) on the surface of the substrate are used as the domain regulating structures. Moreover, although FIG. 1A and FIG. 1B describes the example in which the domain regulating structures are formed on both of the TFT substrate 10 and the counter substrate 20, it is also possible to form the domain regulating structures only on one of the TFT substrate 10 and the counter substrate 20.
Incidentally, the conventional MVA liquid crystal display device causes a phenomenon in which a screen seems slightly whiter when viewed from an oblique angle. FIG. 2 is a graph showing transmittance-applied voltage (T-V) characteristic when viewing a screen from front and T-V characteristic when viewing the screen from above at an angle of 60°, in which the lateral axis indicates the applied voltage (V) and the longitudinal axis indicates the transmittance. As shown in FIG. 2, when a voltage slightly higher than a threshold voltage is applied (a region surrounded by a circle in the graph), the transmittance when viewing the screen obliquely becomes higher than the transmittance when viewing the screen from front. On the contrary, when the applied voltage is increased to a certain level, the transmittance when viewing the screen obliquely becomes lower than the transmittance when viewing the screen from front. For this reason, differences in luminance among a red picture element, a green picture element, and a blue picture element are reduced when viewing the screen from obliquely, and resultantly, the phenomenon of the whiter screen occurs as described previously. This phenomenon is called wash out. Wash out occurs not only in the MVA liquid crystal display device but also in the TN liquid crystal display device.
The U.S. Pat. No. 4,840,460 Specification disclosed a technique to divide one picture element into a plurality of sub picture elements and to subject the sub picture element to capacitive coupling. In such a liquid crystal display device, electric potential is divided depending on capacity ratios among the respective sub picture elements. Accordingly, it is possible to apply different amounts of voltages to the respective sub picture elements. Therefore, one picture element appears to include a plurality of regions having different threshold values in terms of the T-V characteristic. When one picture element includes the plurality of regions having different threshold values in terms of the T-V characteristic, it is possible to suppress the phenomenon that the transmittance when viewing the screen obliquely becomes higher than the transmittance when viewing the screen from front, and thereby to suppress the phenomenon of the whiter screen (wash out). The above-described method of improving a display characteristic by means of dividing one picture element into the plurality of capacitively coupled sub picture elements is called a halftone gray scale (HT) method applying capacitive coupling. Note that the liquid crystal display device disclosed in U.S. Pat. No. 4,840,460 is a TN liquid crystal display device.
FIG. 3 is a plan view showing an example of a TFT substrate in a liquid crystal display device configured to achieve the HT method applying capacitive coupling, and FIG. 4 is a cross-sectional view taken along the I-I line in FIG. 3.
On a glass substrate 51 constituting a base of the TFT substrate, there are formed a plurality of gate bus lines 52 extending in a horizontal direction (an X direction) and a plurality of data bus lines (drain bus lines) 55 extending in a vertical direction (a Y direction). A rectangular region defined by the gate bus lines 52 and the data bus lines 55 constitutes each picture element region. Meanwhile, on the glass substrate 51, there are formed auxiliary capacitance bus lines 53 disposed parallel to the gate bus lines 52 and intersecting the center of the respective picture element regions.
A first insulating film 54 is formed in a space between each of the gate bus lines 52 and each of the data bus lines 55 and in a space between each of the auxiliary capacitance bus lines 53 and each of the data bus lines 55. By using this first insulating film 54, the gate bus lines 52 and the data bus lines 55, and, the auxiliary capacitance bus lines 53 and the data bus lines 55 are electrically insulated, respectively.
A thin film transistor (TFT) 56, a control electrode 57, an auxiliary capacitance electrode 58, and sub picture element electrodes 61a and 61b are formed in each picture element region. As shown in FIG. 3, the TFT 56 applies part of the gate bus line 52 as a gate electrode. Moreover, as shown in FIG. 4, a semiconductor film 56a constituting an active layer for the TFT 56 is formed above the gate bus line 52, and a channel protection film 56b is formed on this semiconductor film 56a. 
A drain electrode 56d of the TFT 56 is connected to the data bus line 55, and a source electrode 56s thereof is disposed in a position opposed to the drain electrode 56d while sandwiching the gate bus line 52. Moreover, the auxiliary capacitance electrode 58 is formed in a position opposed to the auxiliary capacitance bus line 53 while sandwiching the first insulating film 54. Furthermore, as shown in FIG. 3, the control electrode 57 is electrically connected to the source electrode 56s and to the auxiliary capacitance electrode 58 through a line 59.
The data bus lines 55, the TFT 56, the control electrode 57, the auxiliary capacitance electrode 58, and the line 59 are covered with a second insulating film 60, and the sub picture element electrodes 61a and 61b are formed on the second insulating film 60. The sub picture element electrode 61a is capacitively coupled to the control electrode 57 while sandwiching the second insulating film 60. Meanwhile, the sub picture element electrode 61b is electrically connected to the auxiliary capacitance electrode 58 through a contact hole 60a which is formed on the second insulating film 60. Surfaces of the sub picture element electrodes 61a and 61b are covered with an alignment film 62.
In the meantime, as shown in FIG. 4, the counter substrate includes a color filter 72 formed on one of surfaces (which is on the lower side in FIG. 4) of a glass substrate 71 constituting a base, a common electrode 73 formed on the color filter 72, and an alignment film 74 covering a surface of the common electrode 73.
The TFT substrate and the counter electrode are disposed opposite to each other while sandwiching spacers (not shown). Then, liquid crystal 80 is sealed between the TFT substrate and the counter substrate.
In a case of a transmissive liquid crystal display device, the sub picture element electrodes 61a and 61b are made of a transparent conductive material such as indium-tin oxide (ITO). On the other hand, in a case of a reflective liquid crystal display device, the sub picture element electrodes 61a and 61b are made of a highly reflective material such as aluminum.
FIG. 5 is an equivalent circuit diagram showing one picture element in a liquid crystal display device including the above-described TFT substrate. In FIG. 5, reference code CLC1 denotes a capacitance composed of the sub picture element electrode 61b and the common electrode 73, reference code CS denotes a capacitance composed of the auxiliary capacitance electrode 58 and the auxiliary capacitance bus line 53, reference code CC denotes a capacitance composed of the sub picture element electrode 61a and the control electrode 57, and reference code CLC2 denotes a capacitance composed of the sub picture element electrode 61a and the common electrode 73. As shown in FIG. 5, a voltage is divided by the capacitance CLC2, which is formed between the sub picture element electrode 61a and the common electrode 73, and the control capacitance CC. Accordingly, when a voltage applied to the sub picture element electrode 61b is Vpx1, a voltage Vpx2 to be applied to the sub picture element electrode 61a will be calculated by the following formula (1):
                              V          px2                =                                            C              C                                                      C                C                            +                              C                LC2                                              ×                      V            px1                                              (        1        )            
Although an actual voltage ratio (Vpx2/Vpx1) is a design item for a display characteristic of a liquid crystal display device, it is considered ideal that the voltage ratio is in a range approximately from 0.6 to 0.8.
A sub picture element electrode to which a display voltage is applied through capacitive coupling, such as the sub picture element electrode 61a, will be hereinafter referred to as a floating sub picture element electrode. Meanwhile, a sub picture element electrode electrically connected to a TFT through a low-resistance conductive body (a line, for example), such as the sub picture element electrode 61b, will be hereinafter referred to as a sub picture element electrode directly connected to the TFT.
As shown in FIG. 6, Japanese Patent No. 3076938 Specification (equivalent to Japanese Unexamined Patent Publication No. 5(1993)-66412) discloses a TN liquid crystal display device in which a picture element electrode is divided into a plurality (which is equal to four in FIG. 6) of sub picture element electrodes 91a to 91d and control electrodes 92a to 92d are respectively disposed below the sub picture element electrodes 91a to 91d through an insulating film. In this liquid crystal display device, display voltages are applied to the control electrodes 92a to 92d through a TFT 90. Since the sizes of the respective control electrodes 92a to 92d are different from one another, the voltages to be applied to the sub picture element electrodes 91a to 91d are also mutually different. Accordingly, it is possible to obtain an effect attributable to the HT method, namely, an effect to suppress wash out. Here, to avoid leakage of light from a space between any of the sub picture element electrodes 91a to 91d, another control electrode 93 is also disposed in the space between any of the sub picture element electrodes 91a to 91d. 
However, the inventors of the present invention have carried out experiments and researches and have found out that a liquid crystal display device including the above-described conventional floating sub picture element electrodes caused deterioration in the display characteristic attributable to image sticking.
FIG. 7A to FIG. 7C and FIG. 8 are schematic diagrams showing a testing method for measuring a degree of image sticking. Firstly, a black and white checker pattern as shown in FIG. 7A is displayed on a liquid crystal display device continuously for a certain period. Thereafter, a halftone pattern as shown in FIG. 7B is displayed on the entire screen of the liquid crystal display device. In this case, when image sticking occurs on the screen, the checker pattern becomes dimly visible as shown in FIG. 7C. After switching from the checker pattern display to the halftone display, luminance is measured along the X-X line in FIG. 7C, for example. Then, assuming that luminance in a dark portion is “a” and a difference in luminance between the dark portion and a bright portion is “b” as shown in FIG. 8, an image sticking ratio defined as 100×b/(a+b) will be calculated.
An image sticking ratio of a liquid crystal display device not including the floating sub picture element electrodes and an image sticking ratio of a liquid crystal display device including the floating sub picture element electrodes were measured in accordance with the above-described method. As a result, the image sticking ratio of the liquid crystal display device not including the sub picture element electrodes was equal to or less than 5%. On the contrary, the image sticking ratio of the liquid crystal display device including the sub picture element electrodes was equal to or more than 10%.